Another approach towards security is to use laminated glass in the
openings big enough for burgers to get in through. They'll have to
bash in numerous imes to get through, each time making a breaking
glass noise. They soon give up, as every glass smashing noise is a
real risk to being caught. Repeated smashing is a dead give away. But
retrofitting this isnt really a cheapie option.

Yes, a good idea and there is always the option of security film too.

Problem is that if they want to get in a simple large jemmy bar will not
only remove the glass but the whole frame too!

I've mentioned security film there but please be aware that for a proper
result it's not a DIY job. Most first timers mess it up and end up with dull
patches and creases all over the gaff.